1. Field of the Invention
The present invention relates to a liquid supply apparatus and an image forming apparatus, and more particularly to a technique for performing temperature control of a liquid supplied to a liquid ejection head.
2. Description of the Related Art
An inkjet recording apparatus is provided with a recording head (inkjet head) in which a plurality of nozzles are arranged in an ejection plane, and an image is formed on a recording medium by ejecting ink droplets from the nozzles, while the recording medium and the recording head are moved relative to each other. Examples of an ink ejection system of the recording head include a piezoelectric system in which the displacement of a piezoelectric element is used to pressurize the ink inside a pressure chamber and eject an ink droplet from the nozzle, and a thermal system in which thermal energy generated by a heat-generating element such as a heater is used to generate a gas bubble inside a pressure chamber and eject an ink droplet from the nozzle by the generated pressure.
Such inkjet recording apparatuses can be of a serial system or a line system. The serial system is provided with a recording head in which nozzle rows are disposed along the conveyance direction of the recording medium and recording is performed by repeating intermittently the reciprocating movement of the recording head in the widthwise direction of the recording medium (direction perpendicular to the paper conveyance direction; main scanning direction) and conveyance of the recording medium. The line system is provided with a receding head in which nozzle rows are disposed along the widthwise direction of the recording medium and recording is performed by moving the recording medium in the paper conveyance direction (sub-scanning direction) with respect to the recording head. One of the benefits of the line system over the serial system is in that the recording speed can be increased, and the line system can be applied widely to various industrial fields.
An ink supply system (ink supply device) of the inkjet recording apparatus is provided with an ink tank accommodating ink to be supplied to the recording head. The ink tank and the recording head are linked by an ink supply path, and a pump serving as a liquid pumping device is installed in the ink supply path. The ink is supplied from the ink tank into the recording head via the ink supply path by driving the pump.
The viscosity of ink used in the inkjet recording apparatus changes according to temperature. Therefore, when the temperature of ink supplied to the recording head changes, the ink viscosity changes, thereby causing variations in ink ejection characteristics. For example, when the ink temperature decreases, the ink viscosity increases, causing reduction in the ejection amount or decrease in the flying velocity of the ink and creating density unevenness in the recorded image. Accordingly, inkjet recording apparatuses have been heretofore suggested that are provided with a temperature adjusting mechanism for adjusting the temperature of ink supplied to the recording head, with the object of stabilizing the ejection characteristic of the recording head (see, for example, Japanese Patent Application Publication No. 3-104655).
However, since the inkjet recording apparatus described in Japanese Patent Application Publication No. 3-104655 is provided with a temperature adjusting device for each ink color, the cost of the apparatus is raised. Further, since the total amount of droplets to be ejected onto the recording medium is determined, when a temperature adjusting device is provided for each color, the temperature adjustment capability corresponding to the maximum droplet ejection amount is necessary for each color and an excess capability as a whole is required, thereby raising the cost.
In particular, in an inkjet recording apparatus of a line system, increase in the recording speed and increase in quality of recorded images are required together with wide printing. Therefore, the amount of ink consumed by the recording head (ejection amount) is increased and the amount of generated heat also increases due to increase in a drive frequency. Further, since accuracy needed for ink temperature rises, a controllable temperature range of ink is narrowed.
Thus, a strong temperature adjustment capability is needed for the ink supplied to the recording head and a stringent requirement is also placed on control accuracy relating to ink temperature adjustment. The problem is that these requirements cannot be met by the temperature adjustment performed by air cooling, such as used in the inkjet recording apparatus described in Japanese Patent Application Publication No. 3-104655.
Using a water cooling system to adjust the temperature of ink supplied to the recording head can be also considered, but this approach could result in undesirable significant cost increase because the ink temperature is adjusted separately for each recording head.